1. Field of the Invention
The present invention relates generally to sample concentration for analyzing volatile organic compounds in air, water and soils. More particularly, the invention involves a method and apparatus for radiant energy sample heating and temperature control.
2. Related Art
Sample concentrators are used in purge-and-trap, headspace, and thermal desorption gas chromatography ("GC") analysis. Purge-and-trap GC technique has been used for analyzing volatile organics in water since approximately the early 1970's. In 1987 the U.S. Environmental Protection Agency ("EPA") promulgated national primary drinking water regulations for certain volatile organic chemicals ("VOCs"). The EPA also proposed maximum contamination levels for eight volatile organic chemicals. These regulations require the use of the purge-and-trap GC technique. In addition to the eight regulated volatile organic chemicals, the EPA also promulgated monitoring requirements for an additional 52 synthetic volatile organic chemicals.
The EPA has approved certain analytical methods for analyzing these 60 compounds. One of the methods is 502.2, a purge-and-trap capillary-column GC method using a photoionization detector and an electrolytic conductivity detector joined in series. A second method is method 524.2, a purge-and-trap capillary-column GC-MS method.
Purge-and-trap systems for analyzing VOCs in drinking water have been assembled from a variety of equipment typically including a purging device, trap, and desorber. These systems also are referred to as sample concentrators. The purge-and-trap system or sample concentrator interfaces to a GC capillary column, then with a photoionization detector/electrolytic conductivity detector or a mass-spectrometer. These components are interconnected via pneumatic conduits.
Highly volatile organic compounds with low water solubility are extracted (purged) from the sample matrix by bubbling an inert gas (i.e., helium or nitrogen) through a five milliliter aqueous sample. Purged sample components are trapped in a tube containing suitable sorbent materials. When purging is complete, the sorbent tube is heated and backflushed with the inert gas to desorb trapped sample components onto a capillary GC column. The column is temperature programmed to separate the method analytes which are then detected with a photoionization detector (PID) and a halogen specific detector placed in series, or with a mass spectrometer.
Tentative identifications are confirmed by analyzing standards under the same conditions used for samples, and comparing results and GC retention times. Additional confirmatory information can be gained by comparing the relative response from the two detectors. Each identified component is measured by relating the response produced for that compound to the response produced by a compound that is used as an internal standard. For absolute confirmation, the gas chromatography/mass spectrometry (GC/MS) determination according to method 524.1 or method 524.2 may be used.
As stated above, the typical purge and trap system consists of the purging device, trap, and desorber. Systems are commercially available from several sources that meet EPA specifications.
Under EPA specifications, the glass purging device must be designed to accept five to twenty-five ml. samples with a water column at least 5 cm. deep. Gaseous volumes above the sample are kept to a minimum to reduce "dead volume" effects. The purged gas passes through the water column as finely divided bubbles.
The sorbent trap is a tube typically at least 25 cm. long and having an inside diameter of at least 0.105 inches. The trap contains certain sorbent materials which the EPA has specified as 2,6-diphenylene oxide polymer, silica gel, and coconut charcoal. The EPA regulations specify the ratios of the adsorbent material. The desorber must be capable of rapidly heating the trap to 180.degree. C.
The model 4460A sample concentrator manufactured by OI Analytical of College Station, Tex., is an example of a purge and trap, or sample concentrator, device. The model 4460A is a microprocessor controlled device that stores method 502.2 and 524.2 operating conditions as default parameters. Operating conditions may be changed by the user to accommodate other types of purge and trap analysis.
In addition to purge-and-trap methods and analyses, sample concentration gas chromatography is used in headspace analysis of liquids and solids, and in thermal desorption analysis of air tube samples. Headspace and thermal desorption techniques are not only used for environmental analyses, but also for clinical and industrial applications.
EPA standard 502.2 specifies purging 5 milliliters of water per 11 minutes. EPA standard 524.2 specifies 25 milliliters. This time period is an attempt to compromise the optimal purging time for a broad range of analytes, each having different volatility and solubility characteristics. By heating the water sample, it is possible to accelerate the volatility and decrease the solubility of analytes to more completely purge each of the analytes out of solutions during the same time period.
Sample containers, or sparge vessels, are conventionally heated by placing a heater jacket, or pocket heater, around the glass outside surface of the sparge vessel. By contacting the outside surface of the sparge vessel, the pocket heater conducts heat to the sample in the vessel. One example of such a device is the TEKMAR pocket heater, which may be placed around the outside of a sparge vessel.
Another type of heater assembly for sparge vessels is the tube type heater which fits snugly against the outside of the glassware. Pocket heaters and tube heaters are conventionally heated with electric current to the temperature of as high as 100.degree. C.
Although the pocket heater and tube heater are advantageous in that they are inexpensive and simple to use, a problem encountered in their use is delay for transferring heat from the jacket or tube to the inside of the sparge vessel. Even if the temperature of the jacket or tube heater is precisely regulated, that same temperature may not necessarily be reached at the inside of the vessel.
The use of conductive heating of samples has other disadvantages and problems. There is a time delay in first heating the jacket (for example, 5 minutes), and then transferring the heat to the sample (for example, an additional 7 or 8 minutes).
Similarly, with these heating systems it is not possible to heat all samples uniformly. This problem is in part due to the fact that the jacket does not have a uniform fit around the sparge. Typically, a thermocouple is used to monitor the temperature of the jacket. However, even if the temperature of the jacket is known, the temperature of the sample may be significantly lower. Therefore, it may be necessary to compensate for this difference by increasing the jacket temperature.
As a result of these problems, each sample may be at a different temperature, and may be purged at a different rate. This means that interpretation of GC results and detection of analytes is less reliable and consistent from one sparge vessel to the next.